Get a Straight Answer

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Listed below are questions submitted by users of "From Stargazers to Starships" and the answers given to them. This is just a selection--of the many questions that arrive, only a few are listed. The ones included below are either of the sort that keeps coming up again and again, or else the answers make a special point, often going into details which might interest many users.

For a complete list, including earlier questions not listed below, click here. You may also link from here to a listing of questions arranged by topic.

Go to my web site "From Stargazers to Starships" http://www,phy6.org/stargaze/Sintro.htm and read section #16 on Newton and his laws. Then you can go read other sections, e.g.#18A on Newton's 3rd law, #18b on momentum and the recoil of a cannon when fired, and #25 on the principle of the rocket. Recoil and rocket actions are two good examples of Newton's 3rd law; the examples in #18b of the rotary sprinkler, of the boat, and of falling off a log can help, too.

Your experiment with balls does not seem to relate to Newton's law. In any experiment it is important, not just that it works, but that you can explain what is happening. Separating two balls with a spring, held compressed by a string, and then cutting the string, may demonstrate the 3rd law, as in #25. You might find other examples.

Asteroids which have been tracked may be 1-5 kilometers across (the bigger, the fewer). Such an asteroid can cause great damage but will not destroy Earth or its life. Just today or yesterday the press reported a network of 4 large telescopes to survey the skies for objects down to 140 meters wide. Of course, even if one is seen to be heading for a collision with Earth, we still lack the technology to prevent such an encounter.

Smaller chunks of rock, gravel, iron and ice keep hitting Earth. During the daytime on August 10, 1972, a 200-ton meteorite skimmed the upper atmosphere above Utah, leaving a visible trail. It just missed Earth. In 1994 comet Shoemaker-Levy was observed to hit Jupiter. However, such events are rare enough to be ignored.

Velocity means kinetic energy, and at the velocity of the shuttle, that energy is sufficient to melt metal (as unfortunately happened in the "Columbia" disaster). To get rid of its orbital velocity, the shuttle has to give up energy, and the only practical way is by converting it to heat and letting the heat radiate away into space (to reduce speed with rocket thrust, you need a rocket nearly as big as the one used in the launch). Its forward-facing surface therefore must get very hot, which is why a heat-resistant ceramic is used, or a composition that chars away. Furthermore, by putting a large heat shield in front, the spacecraft creates a shock front which is even hotter, from which most of the heat is actually radiated.

Note also that this happens at high altitude, where air is very rarefied: at a lower altitudes--say, that of a jetliner--even if you could tolerate the heating, the force of air resistance may still be sufficient to break the spacecraft. Only after most of the energy is gone can a spacecraft re-enter the atmosphere with its occupants or payload intact. Spacecraft which enter too fast or too steeply are usually destroyed.

http://www.phy6.org/stargaze/Snuclear.htm
and from adjoining sections in its collection (it is also copied there--see index). I can send you a copy if you wish; finding the time teach it (or any other part of the "Flexbook") remains a problem.

At the same time, from what I have seen so far, the chapters of the "Flexbook" are rather disjoint and do not really cover most of "modern physics", certainly not in any systematic way.

(2) The proper answer to teaching modern physics in one year of high school is probably "it cannot be done." Our syllabus is hardly different from that of the 100-year-old text of Millikan and Gale, yet enormous strides in physics were made since then. I suspect physics will not be properly taught in the US until the course is broadened to several years (including middle school), as in Europe. In 1998 I tried to outline the coverage needed, and you will find that outline (submitted to the state of Maryland but not adopted) in

Your coverage of physics seems typical of public schools and ends with a hurried coverage of magnetism. Since time is short, the graduating class may be left with the impression that magnetism is a mysterious property associated with iron, missing the fundamental role of electromagnetism altogether. I tried to introduce magnetism more broadly in "The Great Magnet, the Earth", home page

and in related web collections (see http://www.phy6.org/readfirst.htm) but again,there is much more material than can be available in the skimpy time allocated. Let those few of your students interested in science explore those web sites on their own, and of course, you yourself can pick and choose whatever you fancy.

But as for covering all of physics meaningfully in one year, it can't be done. The most you can do is present a selection, something like a chocolate sampler, and those who want to go further can find material on the web.

Be very cautious with the use of Wikipedia. It often describes what the claimed facts are, but stops short of explaining how and why they were reached, which is really the soul of science!

That is a collection of problems and answers for a short course on nuclear energy, written as a chapter in a free supplementary physics text for schools in the state of Virginia (and anywhere else). Go there to the answers for section S-8A-2 on nuclear binding energy, questions 5 and 6.

In question 5, I ask how much energy is released by fusion of deuterium, and the result corresponds to about 0.5% loss of mass. That energy was largely contained in the neutrons, which are heavier than protons, with which fusion on the Sun starts. The Sun would lose that much of its mass over its hydrogen-burning period of about 10 billion years, through which we are about halfway. The effect on planetary orbits over (say) historical times is thus negligible.

Question 6 compares the mass loss from fusion to mass loss due to ejection of the solar wind. The two numbers are surprisingly close.